Oil refineries typically incorporate one or more different processes for treating and/or converting hydrocarbons, such as, for example, those present in crude oil or other naturally occurring source, to produce specific hydrocarbon products with properties that are useful for particular applications.
To carry out the hydroprocessing operations to treat crude oil and other hydrocarbons to form usable products, oil refineries typically include one or more complexes or groups of equipment designed for carrying out one or more particular treating or conversion processes to prepare desired final products. In this regard, the complexes each may have a variety of interconnected units or vessels including, among others, tanks, furnaces, distillation towers, reactors, heat exchangers, pumps, pipes, fittings, and valves.
Many types of hydrocarbon treating operations are carried out under relatively harsh operating conditions, including high temperatures and/or pressures and within various harsh chemical environments. In addition, due to the large demands for hydrocarbon and petrochemical products, the volumetric flow rate of a hydrocarbon stream through various oil refinery complexes is substantial, and the amount of downtime of the processing equipment is preferably small to avoid losses in output.
High temperature hydrocarbon treating operations generally involve heating a hydrocarbon stream to a process temperature and flowing the hydrocarbon stream through one or more hydrocarbon treating vessels forming a refinery complex. Specific process techniques are utilized depending on the feed and the desired products, and may include flowing the hydrocarbon stream in the presence of other materials and/or reactants, including gases and liquids, adsorbents to remove particular components from the product stream, and/or catalysts to control reaction rates. In this manner, the hydrocarbon stream can be treated to, for example, modify one or more components within the hydrocarbon stream, react one or more components with other materials (e.g. gases) within a vessel, and remove components from the hydrocarbon stream either as potential products, sometimes upon further processing, or for disposal.
Traditionally, austenitic stainless steels have been used to fabricate the oil refinery vessels listed above, because these types of alloys are useful in a variety of harsh environments. The addition of 8% nickel to a stainless steel containing 18% chromium produces a remarkable change in microstructure and properties. The alloy solidifies and cools to form a face-centered cubic structure called austenite, which is non-magnetic. Austenitic stainless steels are highly ductile, even at cryogenic temperatures and have excellent weldability and other fabrication properties.
Many metals, including austenitic stainless steels, can be subject to a highly localized form of corrosion known as stress-corrosion cracking (SCC). SCC often takes the form of branching cracks in apparently ductile material and can occur with little or no advance warning. In low pressure vessels, the first sign of stress corrosion cracking is usually a leak, but there have been instances of catastrophic failures of high pressure vessels due to stress corrosion cracking. Stress corrosion cracking occurs when the surface of the material exposed to a corroding medium is under tensile stress and the corroding medium specifically causes stress corrosion cracking of the metal. Tensile stresses may be the result of applied loads, internal pressure in piping systems and pressure vessels or residual stresses from prior welding or bending.
Austenitic stainless steels can be subject to stress corrosion cracking in, for example, hot chloride solutions, hot caustic soda and hot sulfides or polythionates. Specifically, stress corrosion cracking has been found to occur within refinery complex vessels due to the presence of even small quantities of sulfur content that is either added during a refinery process or is present in the feedstock. The risk of polythionic acid stress corrosion cracking generally increases in temperature ranges of between 370 and 815° C.
In order for polythionic acid stress corrosion cracking to occur in austenitic stainless steels, typically the steel must first undergo sensitization and either concurrently or subsequently be subjected to a corrosive agent, such as polythionic acid. For example, unstabilized grades of austenitic stainless steels such as types 304 and 316, traditionally used in the fabrication of oil refinery complexes, have all exhibited sensitization and polythionic acid stress corrosion cracking due to polythionic acid. Even the stabilized grades such as type 321 and 347 can exhibit sensitization and polythionic acid SCC. Typically, chromium within the austenitic stainless steels reacts with oxygen to form a passive film of chromium oxide that protects the material from corrosion. The passivated metal is able to resist further oxidation or rusting. At high temperatures, however, usually somewhere in the range of between 370 and 815° C. depending on the stainless steel alloy, chromium-rich carbides precipitate out at the grain boundaries. The precipitation of chromium depletes the chromium content adjacent to the grain boundaries forming chromium depleted zones and drastically reducing the corrosion and/or cracking resistance in corrosive environments in these zones. PTA-SCC requires the combination of sulfide scale formation on the metal surface, sensitized microstructure, tensile stress, moisture and oxygen.
FIG. 1, reproduced from D. V. Beggs and R. W. Howe, “Effects of welding and Thermal Stabilization on the Sensitization and Polythionic Acid Stress corrosion Cracking of Heat and Corrosion-Resistant Alloys”, NACE Conference 1993, Paper no. 541, illustrates the temperatures and times at which traditional austenitic stainless steels have been found to exhibit sensitization. As can be seen from the FIGURE, the peak temperatures and times for sensitization of austenitic stainless steels are material specific, although they all generally occur within a temperature range of between 565° and 650° C. Specifically, type 347 stainless steel exhibits peak sensitization at 565° C., (i.e. exhibits sensitization at this temperature faster than at higher or lower temperatures) but does not sensitize at this temperature until after 1,000 hours of being held at the elevated temperature. Type 347 stainless steel is often used in refinery processing equipment due to the longer time that it can withstand sensitization when compared with other stainless steels as shown in FIG. 1. As illustrated in FIG. 1, each stainless steel alloy exhibits a different sensitization envelope, i.e., area on a time/temperature diagram where the alloy exhibits sensitization.
One particularly harsh environment in which austenitic stainless steels are typically observed to undergo stress corrosion cracking is an environment containing halides, usually in the form of chlorides. The presence of chlorides along with an aqueous phase and tensile stresses can result in chloride stress corrosion cracking (“chloride-SCC”) of austenitic stainless steels. This type of cracking is predominantly transgranular and is dependent on time, oxygen, and chloride concentration. Stress corrosion cracking due to chlorides is usually observed in areas of austenitic stainless steels subjected to tensile stresses in the presence of chlorides, oxygen. In general, chloride-SCC will occur where high concentrations of chlorides are present, but may occur in lower concentrations at elevated temperatures. In addition, while high temperatures may reduce the amount of time required for a particular chloride concentration to result in chloride SCC, often lower temperatures cause chlorides to condense on surfaces increasing the concentration of the chlorides on the surfaces. Thus, chloride SCC can be problematic at many temperature ranges. For example, chloride-SCC can occur where chloride concentrations are able to build up, for example by pitting or crevice corrosion of the material surface or on heated surfaces or where chlorides present in the environment condense on a material surface. Chlorides are able to penetrate the passive film to allow corrosive attack of the material to occur. One particularly problematic area of chloride SCC is in condensers where chloride condenses and concentrates on surfaces of the vessel.
Another type of harsh corrosive environment to which sensitized stainless steels are particularly susceptible is one that contains polythionic acid (PTA) formed from the decomposition of sulfide scale by moisture in air. Due to the high temperature of operation and the presence of sulfur (S) and hydrogen sulfide (H2S) in a reducing environment or in a feed stream in many oil refinery complexes and/or processes, an iron sulfide scale can form on stainless steel surfaces. Upon shutdown of the equipment, if the sensitized stainless steel is exposed to moisture and oxygen from the surrounding environment, there is the potential that the metal can crack as a result of polythionic acid stress corrosion cracking (PTA-SCC). In other words, the sulfur and hydrogen sulfide will react with oxygen and moisture from the ambient environment to form polythionic acid. Due to the existence of the chromium depleted zones formed by sensitization, the PTA can attack these zones causing corrosion and ultimately PTA-SCC where the vessel is put under tensile stresses either by being pressurized or by having residual stresses from, for example, welding during fabrication.
Commercially, internal surfaces of refinery complex equipment for carrying out processes at elevated temperatures are usually made of Type 304 and Type 347 austenitic stainless steels, especially for use in sulfur or H2S-containing reducing environments, such as for example hydroprocessing and hydrocracking reactors, heaters and heat exchangers, complexes for converting of liquid petroleum gas (LPG) into aromatics through dehydrocyclodimerization, and processes for catalytic dehydrogenation for the production of light olefins from paraffins. The most widely used stainless steel is probably Type 304, sometimes called T304 or simply 304, because of cost. Type 304 stainless steel is an austenitic steel containing 18 to 20% chromium and 8 to 10% nickel. This and other specialty austenitic stainless steels have been used in these applications due to the high temperature H2S, sulfur, and chloride-SCC corrosion and high temperature hydrogen attack issues that are present in these processes.
In some instances, protective coatings are applied to protect the outside of stainless steel vessels from exposure to chlorides in insulating jackets. In other applications, post welding heat treatment can be used to relieve residual stress in the steel alloys. The risk of PTA-SCC and chloride-SCC in oil refinery equipment has heretofore primarily been addressed by known processes to either prevent the formation of PTA and/or presence of chlorides or to neutralize the PTA in the environment prior to exposure to air.
To reduce the affects of chloride-SCC, precautions are typically taken to minimize the amount of chloride in the process material or feed that will come into contact with austenitic stainless steel equipment. For example, a particular process may utilize a high chloride feed. In addition, precautions are taken to limit the chloride content to low levels in any flushing, purging, or neutralizing agents used in the system.
Preventing PTA formation can be accomplished by either eliminating liquid phase water or oxygen, since these are the components responsible for reacting with the sulfide scale to form the PTA. One approach is to maintain the temperature of the austenitic stainless steel equipment above the dew point of water to avoid condensation of the moisture. Another approach is to purge the equipment with a dry nitrogen purge during any shutdown or startup procedure, when the system is depressurized and the equipment is opened and exposed to air, since this is generally the only time when significant amounts of oxygen might enter the system.
On the other hand, PTA that has or is likely to form within a complex or vessel may be neutralized by an ammoniated nitrogen purge or an aqueous solution of soda ash. In the case of utilizing an ammoniated nitrogen purge, special procedures are utilized to form the ammoniated nitrogen, which is pressurized and blown into the system. On the other hand, a soda ash solution neutralization step involves completely filling the piping or piece of equipment involved with the solution and allowing the equipment to soak for a minimum of two hours prior to exposing the system to air. Each of these processes is time consuming and impractical during the operation of an oil refinery complex as it requires additional materials and additional downtime of the particular equipment to perform the purge or neutralization steps. In addition, due to the presence of the nitrogen, ammoniated nitrogen, or soda ash, special precautions must be taken to protect service workers working on the equipment when these materials are present. Also the removal of these chemicals reduces the need for special handling and waste disposal. If trace levels of the chemicals remain, which is often the case, catalyst in the reactor can be poisoned.
In addition, chemically stabilized austenitic stainless steels like TP321 and TP347 have been used in reactors that process sulfur and chloride containing streams because of their resistance to high-temperature corrosion. However, such austenitic stainless steels are also susceptible to PTA-SCC as a result of exposure to polythionic acid, since it is just a matter of time at temperature for them to sensitize, which falls within the operating conditions of many hydrocarbon treatment processes. Similarly, these materials are susceptible to chloride-SCC through exposure to chlorides at sufficient times and temperatures. Although TP321 and TP347 are generally used in applications according to the above methodologies in petroleum refinery industries, the need for post-weld heat treatment and for special procedures during shutdown and startup of a refinery complex affect not only costs but also production time since they take a certain amount of time to carry out.
There is a continuing need, therefore, for improved processes for treating hydrocarbon streams while avoiding expensive, time consuming and inconvenient additional steps for purging or neutralizing the internal environment to avoid forming polythionic acid and reducing the presence of chlorides within hydrocarbon treating vessels and causing PTA-SCC and chloride-SCC.